Industrial textiles used in filtration, conveyance and similar continuous processes have typically been manufactured by interweaving synthetic yarns according to a selected weave pattern to provide either the entire fabric, or only a base body portion which may subsequently be either encapsulated (e.g. with polyurethane or other similar rugged material) or needled to attach a nonwoven batt material. Such fabrics have been satisfactory for these uses, but the cost of their production is high, particularly when the fabrics must be finely and precisely woven using relatively small yarns and large industrial looms. Further, these fabrics must be rendered endless in some manner, either by installing a seaming element at their opposed longitudinal ends, or by re-weaving the longitudinal yarns back into the fabric structure to form seaming loops or similar joining means, for secure connection by a pintle, coil or similar securing means. It is also known to weave such fabrics in an endless manner, to form a continuous tube, or to assemble successive turns of a narrow woven or nonwoven strip arranged in side-by-side relation to build up a desired length and width of finished fabric.
It will be appreciated that these fabrics are expensive and time consuming to produce, and require a high capital investment in industrial looms and related equipment for subsequent processing, as well as a skilled workforce to operate the equipment and produce an acceptable finished product. The fabrics themselves must be highly stable, resistant to abrasive wear and both chemical and thermal degradation caused by the environments in which they are used.
Various proposals have been made to reduce the manufacturing costs and technical difficulties associated with making these woven textiles by creating them using nonwoven materials. For example, it is known to produce industrial fabrics from one or more layers of film or other continuous media, and to provide perforations, profiling or other treatment prior to assembly. See for example WO 2010/088283, WO 2010/088280, WO 2010/068778, WO 2010/068765, WO 2010/030570, WO 2010/030547 and WO 2010/030298, all commonly assigned to Albany International Corp., which disclose various industrial fabrics formed from continuous strips of a film and which are intended for use in the manufacture of various sheet-like products. In general, the fabrics disclosed in these references are comprised of narrow thermoplastic polymeric strips, such as industrial strapping material, which have been laser drilled to impart porosity.
Other paper machine clothing into which a film-like component has been incorporated are described in various patents, for example U.S. Pat. No. 6,071,837 (Crook), U.S. Pat. No. 6,436,240 (Jeffrey), U.S. Pat. No. 6,531,418 (Lidar et al.), U.S. Pat. No. 6,989,080 (Hansen), and U.S. Pat. No. 7,323,087 (Beck). Other apertured films, and processes for their manufacture, to provide textiles for various industrial applications, are known. For example, U.S. Pat. No. 4,842,794, U.S. Pat. No. 5,207,962, and U.S. Pat. No. 5,262,107 all to Hovis et al., disclose fabrics and processes for forming the same including an apertured thermoplastic film which has been deformed by means of a patterned embossing roll. U.S. Pat. No. 3,162,567 (Heller) discloses a papermakers forming fabric made from a sheet of metal or film which has been punched to form arched portions which extend out of the plane of the sheet to provide drainage openings which are at an angle to the plane of the sheet.
However, none of the references noted above discloses a fabric having the advantageous properties of woven fabrics intended for industrial filtration and similar uses, and which are constructed from two or more layers of a polymer film, each of which has been contoured by providing a combination of an embossing or similar process to raise portions of the film above its general plane to form protrusions, and slit to create apertures at the raised portions.
It would therefore be highly desirable to provide a simple, relatively less expensive process for the use of polymer films for the construction of nonwoven textiles possessing most or all of the advantageous properties of the known woven industrial fabrics, without the disadvantages and cost of the steps required to produce such woven fabrics.
WO 2011/069259 (Manninen) discloses the use of selectively slit and embossed films in the manufacture of industrial textiles. As disclosed, the fabrics are comprised of at least two layers of planar polymeric film each of which is selectively slit and embossed to impart surface contouring and other properties similar to those resulting from various weave designs for woven fabrics. According to the method of manufacture disclosed in the '259 document, the film is pressed by application of heat and pressure using a chosen mold pattern so as to selectively and permanently deform portions out of plane; a slitting process is used to create apertures at the deformations to allow the passage of fluids such as air and water through the fabric. The length of the individual slits is equal to the length of the aperture formed in the film. The document also discloses that a two layer fabric can be provided in which a first layer of a suitably slit and embossed film is mated with a second similarly imprinted sheet so that the outer surfaces of the deformations formed by the embossing processes face one another.
FIGS. 1 to 4 show a prior art film for use in a fabric such as is described in WO 2011/069259 (Manninen). As shown in FIGS. 1 and 2, a polymeric film 12 is selectively deformed and slit to provide protrusions 50 having top surfaces 51 and angled sides 52; the protrusions 50 define apertures 40 and are spaced apart such that flat land areas 60 exist between them. FIG. 2 provides a cross-sectional view of the protrusions 50, their top surfaces 51 and angled sides 52, and land areas 60 shown in perspective in FIG. 1. FIG. 3 shows a detailed view of an assembled film fabric according to the '259 document. As shown in FIG. 3, two similarly slit and profiled film layers 80 and 90 are joined together such that the top surfaces 51 of a plurality of protrusions 50 of one layer such as 80 are in contact with land areas 60 of second film layer such as 90. The two film layers 80 and 90 are then bonded such as by welding to one another to form the eventual completed fabric which is arranged in the manner shown in cross-section in FIG. 4. As can be seen in this Figure, the two film layers 80 and 90 are joined to one another and spaces such as 70 are maintained and defined between them by the angled sides 52 of the protrusions 50. These angled sides 52 may provide adequate support to keep the film layers 80 and 90 apart so that the openings such as 70 are maintained, but this structure may have less than desirable resistance to repetitive compressive loading, in particular because there is no material remaining at the sides (i.e. along the sides of apertures 40) of the protrusions to support the top surfaces 51 and angled sides 52; it has been entirely removed by the slitting process and so there is no material reinforcement. Further, the shape of angled sides 52 is such that there is no inherent support in them other than the bend where angled side 52 transitions to top surface 51. In addition, there is no simple means to adjust the number of openings in the film, and their size, without changing the embossing pattern which is both costly and time consuming.
Fabrics such as are disclosed in the '259 document have been constructed, tested and found to be generally satisfactory for use in the applications for which they are intended. However, in certain situations, where the textile is to be exposed to repetitive compressive loading while in operation, it may be desirable to augment its ability to resist such compression and thereby maintain void volume between its layers, as well as permeability. If the protrusions 50 are compressed or flattened, the apertures 40 may become restricted, thus altering the permeability and other desirable fabric properties. It would also be advantageous if the permeability of the fabric could be easily adjusted so as to suit exactly the requirements of the application for which it is intended.
It would also be advantageous to provide a protrusion design which either builds in support to angled sides 52 (i.e. the support is inherent in its shape) or retains some material to buttress top surfaces 51 of the protrusions 50, combined with a slitting pattern which is adjustable as required in either or both the longitudinal and cross directions so as to impart a desired level of permeability to the resulting film fabric structure.
It has now been found that, in the slit and embossed prior art film fabrics, the stiffness (resistance to compression) of the protrusions can be significantly improved by either replacing the rectilinear configuration of the prior art design with curved surfaces or shells, or by changing the size and/or orientation of the slits 30 so as to leave behind material to support the sides and tops of the protrusions, while easily imparting a desired permeability to the fabric in a manner that does not detract from desired physical properties, and does not require a change in the molding pattern used to form the protrusions.
As used herein, the term “shell” is used in its structural sense and generally refers to an essentially two-dimensional plate which has been curved in at least one dimension, or as many as three. A primary difference between a shell structure and a plate structure is that, in the unstressed state, the shell structure has curvature as opposed to plate structures which are flat. Where a flat plate acts in a similar manner to a beam with bending and shear stresses, shells are analogous to a cable which resists loads through tensile stresses. Shell structures are common in nature and structural engineering, and include familiar objects such as eggs, ships' hulls or domes. Shell structures which provide effective support when a line of thrust acts on their curved cross-section are of particular interest as they have appreciable bending stiffness. These out-of-plane regions are shaped such that: (1) the depth of the cross section is increased giving increased area moment of inertia, I; (2) the depth of cross section is continuously varied along the length of the beam/arch; and (3) curved beams or arches are produced to reduce the compressive deflections of the out-of-plane region. The bending stiffness, EI, of a member increases with increasing area moment of inertia, I. So, in general, the greater the curvature of the shell, the more effectively it will resist compressive loading due to its inherent bending stiffness. The present invention utilizes in part the structural benefits provided by the curved profiles of shells in the design of walls of the protrusions so as to enhance their resistance to compressive loading by increasing their bending stiffness; this will in turn assist the fabric to maintain void volume while under compressive load.
The present invention addresses and provides an effective solution to the above and other difficulties and limitations of the prior art.